The present invention relates to novel polyether polyols based on sucrose and formitol. These polyols have a relatively low viscosity and are particularly suitable for the production of rigid, closed-cell polyurethane foams.
Rigid, closed-cell polyurethane foams are generally obtained by reacting a polyether polyol with an aromatic polyisocyanate, such as crude 4,4'-diisocyanato diphenyl methane. Such polyurethane foams are particularly suitable for the production of insulating materials, sandwich structures, building panels and numerous other building elements.
The physical and mechanical characteristics of these foamed plastics depend to a large extent upon the structure and molecular size of the polyethers which are used. Polyethers derived from trihydric alcohols, such as trimethylol propane or glycerin, have been used to make such rigid foams. However, polyurethane foams obtained from such polyethers have poor dimensional stability. Increasing the amount of cross-linking may improve the quality of such rigid polyurethane foams; however, the thus-obtained products are still unsatisfactory.
One approach to improving rigid polyurethane foams which has been investigated is the use of high-functional polyether polyols based on sucrose. However, these efforts have not been too successful due to the number of technical problems encountered with conventional processes for producing sucrose polyether.
It is known to react sucrose with alkylene oxide in an aqueous solution in the presence of sodium hydroxide and ethylene oxide and subsequently convert the product into sucrose hydroxy alkyl ethers (J. W. LeMaistre R. B. Seymour, J. Org. Chem. 13,782 (1948)). In one process based on this reaction, sucrose is reacted at elevated temperatures with ethylene oxide or propylene oxide in a concentrated aqueous solution in the presence of a potassium hydroxide catalyst (U.S. Pat. Nos. 3,055,085 and 3,153,002; German Pat. No. 1,443,026). However, such processes are subject to undesirable secondary reactions, such as partial hydrolysis of the alkylene oxide by the water used as the reaction medium. When such hydrolyzed alkylene oxides are used to form polyalkylene glycols, the reaction mixture becomes very darkly colored. This dark coloring has a disadvantageous effect upon the properties of polyurethane foams which are produced from these sucrose hydroxy alkyl ethers. These secondary reactions also result in a large amount of bifunctional, linear by-products which reduce the functionality of the product polyethers (as compared to the functionality of a pure sucrose polyol). As a result of the large proportion of such by-products, the product sucrose polyethers have limited suitability for the production of satisfactory polyurethane foams. In fact, such by-product laden polyethers tend to yield brittle foams of moderate strength having a non-uniform cell structure. Another disadvantage of polyurethane foams obtained from such sucrose polyethers is the low proportion of closed cells and consequent poor heat insulation ability.
For these reasons, it has been attempted to alkoxylate sucrose in the presence of xylene (U.S. Pat. No. 2,652,394) rather than water. However, this process yields highly discolored products as a result of converting the sucrose into caramel or carbonizing the sucrose.
It has been found that the production of large quantities of bifunctional by-products may be reduced by a process wherein sucrose is first reacted with from 4 to 8 mols alkylene oxide in a concentrated, aqueous solution in the presence of potash lye. Substantially all of the water is then removed from the reaction mixture and more alkylene oxide is added (German Pat. No. 1,443,022). However, since a large quantity of the alkylene oxide is reacted in the presence of relatively large amounts of water, the disadvantages already mentioned with respect to the process of alkoxylation without dehydration, also occur to a considerable extent in this process. One of the disadvantages of these known processes which employ an aqueous solution is that most of the sugar must be added at temperatures which are approximately the same as the boiling point of water or only slightly below. Another disadvantage of such processes is that the rate of the alkylene oxide addition reaction is relatively slow which greatly promotes the formation of by-products.
The combination of the required characteristics for rigid polyurethane foams (i.e. a fine cell feature, dimensional stability in moist heat and at cold temperatures) with outstanding flow behavior in the foaming process and the required curing behavior (particularly curing rate and formation of a hard peripheral area with an optimum adhesion to covering layers), cannot be achieved by using either known polyether polyols based on sucrose or sucrose glycerin mixtures or sorbitol as the starting material.
U.S. Pat. No. 2,990,376 discloses that a polyol which is suitable for the production of rigid polyurethane foams is obtained if small amounts of a particular sucrose polyether are added to a glycerin polyether. Another process for the production of rigid polyurethane plastics is suggested in German Auslegeschrift No. 1,285,741. In this process, polyethers which contain a carefully controlled ratio of sucrose and glycerin are used. The foams obtained from either of these polyethers do exhibit a good dimensional stability, but do not have the other required characteristics of a suitable rigid polyurethane foam (particularly short in-mold times and the ability to form hard surfaces with outstanding adhesion to covering layers). Additionally, use of relatively expensive glycerin as the starting material for polyethers is economically impractical.
German Offenlegungsschrift No. 2,639,083 discloses a method for the preparation and use of formitol-polyethers. Polyethers which are prepared according to this process are advantageously distinguished from sucrose polyethers and glycerin-started polyethers, particularly by improved flowability. However, when used in the production of polyurethane foams, they do not possess all of the above-mentioned desirable characteristics (see Comparative Examples 10 to 16 infra).